Lightning arrester with the frustoconical surface of the parallel electrodes being exposed toward each other at the narrow end of a diverging spark slot



Sept. 16, 1969 7 K. A. DOOLEY ErAL 3,467,877

LIGHTNING ARRESTER WITH THE FRUSTOCONICAL SURFACE OF THE PARALLEL ELECTRODES BEING'EXPOSED TOWARD EACH OTHER AT THE NARROW END OF A DIVERGING SPARK SLOT Filed Aug. 16, 1966 Fly;

ffzlsey ,4, aoazzy W/( 1 041W 4. Foe/(Had 04 W0 1. J'WOIJ [A R4 .6, 77V0M 9J 94.: P// M 1 4/1 f0 :szn/ J? INVENTORS 0min, BWZVMW United States Patent US. Cl. 313325 12 Claims ABSTRACT OF THE DISCLOSURE In a narrow but wide slot in the insulating material a lightning arrester gap is formed between the tip portions of two electrodes arranged in parallel. The axes of the electrodes are offset from the plane of the slot and the electrodes are exposed into the slot only by a small portion of the frustoconical surface forming the chamfer. The edges of the slot approximately coincide with the portions of the electrodes nearest one another and extend away therefrom in slightly diverging angles to an open end of the slot. The insulating material is gas evolving material and it is reinforced with a stainless steel band under tension. An insulating tube for one electrode extends well beyond the end of the other electrode. The inserted portions of the electrode are circumferentially grooved to help insure their retention in place when pressed into the preformed insulating member.

INTRODUCTION The invention of which this disclosure is offered for public dissemination if adequate patent protection is available relates to a lightening arrester. It is adapted particularly for use as a surge relief protection for watthour meters.

When lightning strikes a primary power line it creates a power surge in the secondary distribution lines connected thereto. In some instances much the same effect can result merely from the proximity of a charged cloud to a power line, even though a discharge from the cloud does not actually strike the line. Surges in the secondary line may occur even though a transformer is between the primary and secondary lines, and the primary line is protected by a lightning arrester. The term power surge as used herein refers to the secondary line effects resulting at least semidirectly from the lightning stroke usually on the primary line, and does not refer to the flow of current from the main power source. The later effect is designated power follow. The power surge in the secondary lines often will be of a magnitude which will damage watt-hour meters connected thereto. Even when the surge is not of this magnitude, damage may result from the power follow if not promptly checked. The term lightning arrester as employed herein is used to refer to devices for protection against both power surge resulting directly or semidirectly from lightning strokes on the associated supply lines as well as for preventing power follow after the lightning stroke.

Obviously, the magnitude of the power surge in the secondary lines will vary from case to case depending upon a number of factors. A primary factorvwill be the magnitude of the voltage surge in the main power line. Although the period of power surge discharge across a lightning arrester is brief, the discharge will create an electric path of ionized gas that is likely to sustain further current flow-the power followfrom the electric power normally present in the power lines. Power follow has proven to be the greatest problem in devising lightning arresters or surge current protective devices for watt-hour meters. Even though the power surge resulting from the lightning stroke inherently may disappear with extreme rapidity, the ionized gas across the gaps of present simple lightning arresters will continue to carry current supplied by thenormal electric power connection. Prior to the invention of James W. Milligan which is the subject of application, Ser. No. 205,264, filed June 26, 1962, some concepts of which may be used in the present invention, such devices were considered praiseworthy if they would end power follow with the occurrence of the next natural voltage zero. The present invention, however, consistently extinguishes power follow within approximately one-eighteenth of a cycle.

As the voltage of the secondary distribution line (or of the line to be protected) is increased, the greater has been the problem in providing an apparatus for suitable protection. At volts it was not particularly diflicult to produce an arrester which could usually extinguish an are upon the occurrent of one or two natural voltage zeros after the termination of the power surge. Upon the occurrence of a natural voltage zero, enough ionized gas is dissipated so that with relatively low voltage present an arc is not often reestablished in the next half-cycle. Substantially more difficulty, however, was encountered in endeavoring to protect watt-hour meters connected to 240 volt lines, even to this imperfect degree.

An important feature of the present invention is that embodiments thereof will withstand more numerous repeated lightning-created surges than the best pertinent prior devices (those of Milligan) without impairment of their ability to operate satisfactorily upon the occasion of the next surge. Although in average surroundings there is little likelihood of a surge occurring many times in the same meter, in a location where power lines are unusually exposed to lightning bolts, a number of surges may occur in a given meter over a period of years. Many surge protection devices completely lose their ability to operate satisfactorily after a few such surges have occurred. Others diminish in their ability to prevent power follow, as originally intended, after a number of surges have occurred.

As compared to the original forms of the Milligan invention, the present invention achieves a surprising further reduction of erosion and hence can arrest a larger number of discharges before losing its ability to substantially prevent power follow. It also is less subject to axial expulsion of the electrodes resulting from forces created by the discharge, and its unique design distributes the explosive forces in such a way that dependably long life can be achieved with considerable saving of material and space. These advantages are achieved largely by a combination of chamfering the terminal ends'of the electrodes, shaping the slot, and so relating the slot to the electrodes that the desired distribution and direction of forces can be obtained. The slot is very narrow throughout in the direction transverse to the plane of the discharge, and it flares out in the plane of the discharge as the distance from the electrodes increases. By this simple arrangement, there is simultaneously achieved a reduction in the electrode exposure, reduction in the axial component of thrust exerted on the electrodes, reduced normal components of force on the walls of the slot, and an outward directing of the explosive forces in such a way that containment is possible.

More complete objectives and advantages of the invention will become apparent from the following description and from the drawings:

DESIGNATION OF FIGURES FIGURE 1 is a longitudinal cross-sectional view of the form of invention chosen for illustration;

FIGURE 2 is an end view as seen from the left of FIG. 1;

FIGURE 3 is an end view as seen from the right of FIG. 1;

FIGURE 4 is a side view of the grounding electrode; and,

FIGURE 5 is a side view of the electrode to be connected to the protected circuit.

Although the following disclosure offered for public dissemination is detailed to ensure adequacy and aid understanding, this is not intended to prejudice that purpose of a patent which is to cover each new inventive concept therein no matter how others may later disguise it by variations in form or additions or further improvements. The claims at the end hereof are intended as the chief aid toward this purpose; as it is these that meet the requirement of pointing out the parts, improvements, or combinations in which the inventive concepts are found.

GENERAL DESCRIPTION The form of the invention chosen for illustration includes a grounding electrode 11, an electrode 12 to be connected to the circuit to be protected, a body of insulation 13 in which the electrodes are embedded and which has a slot 14 to which both electrodes are exposed for discharge between the electrodes. A reinforcing band 16 protects the insulating body 13 from being burst by the currents from a heavy stroke of lightning. In accordance with the teachings of the Milligan patent application, Ser. No. 205,264, the slot 14 is preferably narrower than the diameter of the electrodes. It is also preferably located nonsymmetrically with respect to the axial plane of the electrodes 11 and 12, as seen in FIG. 3. The disclosure of said application is incorporated herein by reference, the present disclosure prevailing, however, on points of difference.

THE PRESENT INVENTION According to the present invention, the terminal ends of the electrodes 11 and 12 are chamfered, and the slot 14 is so shaped and located as to expose only a small area of chamfered surface. At present, it is preferred that the exposed surface be approximately at 45 degree chamfer. This has been found to be satisfactory, but applicants are also confident that any angle within the range of 30 to 60 degrees would also be satisfactory. Preferably the chamfer is uniform around the electrodes so as to form a surface of conical nature, more specifically frustoconical. This is not only a matter of convenience in manufacture, but it also causes the exposed surface in the offcenter slot to be tilted with respect to the plane of the slot. Inasmuch as the off-center location tends to cause the discharge from the electrodes to impinge predominately against one surface, namely the left surface as seen in FIG. 3, this disposition of the exposed surface tends to heighten this impingement effect, and furthermore makes the exposed surface relatively perpendicular to the apparent angle of predominant discharge. The impingement effect can be produced in other ways, at least some of which Milligan has recognized, as by using a nonflat slot shaped and positioned to impose a wall in the natural path of the are between the electrodes. The present invention may minimize the deterioration due to errosion, however. One advantage of the impingement effect is in increasing the liberation of an arc quenching gas, the insulating block 13 preferably being formed of a material which will yield such gas. The material preferably employed is polymerized acetaldehyde resin such as that sold under the name Delrin. The vulcanized fiber sold as Horn Fiber is also suitable, but is less preferred 4 because of being affected by high humidity found in some locations. An alumina hydrate, Al(OH) filled polyester resin is also suitable, although less desirable, especially if numerous surges are likely tobe encountered, because it erodes more rapidly than the preferred material.

The edges 18 of the discharge slot or arc chute 14 preferably extend from the tips of the chamfered areas on the sides of the electrodes generally toward one another at mildly diverging angles as seen clearly in FIG. 1.

The reinforcing band 16 is preferably stainless steel. Both it and the outer surface of insulating block 13 are provided with substantially identical axial tapers, and the band 16 is slightly undersized, so that when the band is driven or pressed on to the approximate position shown, it will be elastically tensioned for effective reinforcement of the insulating body. Furthermore, the shape of the slot 14 leaves maximum strength in the insulating body at the point of greatest need, adjacent the electrodes.

DIMENSIONS FOUND SUITABLE Although the dimensions may be subject to considerable or even wide variation within the principles discussed in this application, it may be helpful to give the following dimensions which have been found to be satisfactory for protection of a 240 volt watt-hour meter. In most instances, manufacturing tolerances are contemplated of plus or minus .002 inch.

All dimensions are given in inches. Electrode diameter .125. Diameter for receiving them, .122, with a tolerance of only .001. Spacing between electrode recesses, .080. Overall length of insulating body, .837. Overall length aside from longer electrode receiving sleeve, .515. Depth of discharge slot 14 at the center, .34. Overlap between this slot and electrodes, .050. Electrode slot radius at base, .085. Taper from this radius to maximum slot edge to edge dimension of .260. Slot width, .014. Body diameter at two ends of the .515 length, .425 and .390, corresponding inside diameters of reinforcing sleeve of .500 length, .420 and .385. Both tapers should be uniform to provide support for the insulating body at all points. Displacement of center line of slot from axial plane of electrodes, .030. Length of 45 degree taper at ends of electrodes, .040. The shape should be substantially matched by the contour of the electrode recesses which in turn are barely intersected, and closely matched along the chamfer by the radiused end of the discharge slot.

ACHIEVEMENT The various factors mentioned have contributed to the provision of a lightning arrester with considerably improved performance and life in spite of its very small size and ease of manufacture. Power follow is held to a low crest value and is extinguished within about one-eighteenth of a cycle (with a 60-cycle power source), and this ability is retained through numerous arresting actions.

Only very gradual deterioration occurs, even when the tests are severe enough for a single one to have largely destroyed the usefulness of prior devices. The small exposure of electrode surface, together with the angle of forces thereon, has reduced the axial force on the electrodes making practical the mass production of the insulating bodies and the electrodes separately and pressing the electrodes into the insulating body with confidence that they will not be displaced. High dependability against bursting, or cracks due to bursting forces, has been achieved.

Numerous tests have disclosed satisfactroy operation of this device, as disclosed in detail, when connected across a power source of 305 volts and having a short circuit capacity of 26,000 amperes. These tests have included surges up to 10,000 amperes, a level which these devices will rarely be called upon to pass in actual service. Previously available simple low cost spark gaps have not approached this performance. It is intended that by suitable adjustment of the slot parameters within the teachings here disclosed, the devices will be made suitable for use with operating voltages of at least 480 volts.

We claim:

1. A surge current protective device including: an insulative body having walls defining a slot with an open side, said slot having a narrow width as measured between two opposite walls; a pair of rod shaped electrodes embedded in said body and having externally chamfered terminal ends with the chamfered portions exposed partially toward each other in said slot opposite said open side, said electrodes having a width, as measured parallel to said width of said slot, that is at least as great as said width of said slot, said electrodes having conductor portions adjacent said ends, each with a longitudinal axis approximately parallel to the axis of the other, said conductor portions being side by side with said axes approximately parallel to the slot and offset to one side of a plane midway between said two walls. I

2. A surge current protective device inclduing: an insulative body having walls defining a slot with an v.open side, said slot having a narrow width as measured between two opposite walls; a pair of rod shaped electrodes embedded in said body and having externally chamfered terminal ends with the chamfered portions exposed partially toward each other in said slot opposite said open side, said electrodes having a width, as measured parallel to said width of said slot, that is at least as great as said width of said slot, said electrodes having portions adjacent said ends, each with a longitudinal axis approximately parallel to theaxis of the other.

3. A surge current protective device including: an insulative body having walls defining a slot with an open side, said slot having a narrow width as measured between two opposite walls; a pair of rod shaped electrodes embedded in said body and having externally chamfered terminal ends with the chamfered portions exposed partially toward each other in said slot opposite said open side, said electrodes having a width, as measured parallel to said width of said slot, that is at least as great as said width of said slot said electrodes having portions adjacent said ends, each with a longitudinal axis approximately parallel to the axis of the other, the electrodes being insulated from one another by said body except where the chamfered surface is exposed in the slot.

4. A surge current protective device including: an insulative body having walls defining a slot 'with an open side, said slot having a narrow width as measured between two opposite walls; a pair of rod shaped electrodes. embedded in said body and having externally chamfered terminal ends with the chamfered portions peripherally around each, forming surfaces of substantially conical nature, having portions on one side of each exposed partially toward each other in said slot opposite said open side, said electrodes having a width, as measured parallel to said width of said slot, that is at least as great as said width of said slot, said electrodes having portions adjacent said ends, each with a longitudinal axis approximately parallel to the axis of the other, said portions being side by side with said axes approximately parallel to the slot and offset to one side of a plane midway between said two walls.

5. A surge current protective device including: an insulative body having walls defining a slot with an open side, said slot having a narrow width as measured between two opposite walls; a pair of rod shaped electrodes embedded in said body and having externally chamfered terminal ends with the chamfered portions peripherally around each, forming surfaces of substantially conical nature, having portions on one side of each exposed partially toward each other in said slot opposite said open side, said electrodes having a width, as measured parallel to said width of said slot, that is at least as great as said width of said slot, said electrodes having portions adjacent said ends, each with a longitudinal axis approximately parallel to the axis of the other.

6. A surge current protective device including: an insulative body having walls defining a slot with an open side, said slot having a narrow width as measured between two opposite walls; a pair of rod shaped electrodes embedded in said body and having externally chamfered terminal ends with the chamfered portion exposed partially toward each other in said slot opposite said open side, said electrodes having a width, as measured parallel to said slot, that is at least as great as said width of said slot, said electrodes having portions adjacent said ends, each with a longitudinal axis approximately parallel to the axis of the other, the electrodes being insulated from one another by said body except where the chamfered surface is exposed in the slot; and the slot having its lateral edges diverging from the respective chamfered exposed portions.

7. A lightning arrester for protecting electrical apparatus including a pair of electrodes extending generally parallel to one another from their terminal ends and with terminal end portions on at least adjacent sides thereof having outer surfaces sloping approximately 30 to 60 degrees with respect to the axial direction, and a body of insulating material having a discharge slot therein extending between said surfaces and away therefrom, opening at one side of the body, and formed by narrowly spaced opposing surfaces, the intersections between the recesses for the electrodes and the slot being constricted to expose to the slot only a small portion of the cross-sectional area of each electrode.

8. A lightning arrester for protecting electrical apparatus including a pair of electrodes extending generally parallel to one another from their terminal ends and with terminal end portions on at least adjacent sides thereof having outer surfaces sloping approximately 30 to 60 degrees with respect to the axial direction, and a body of insulating material having a discharge slot therein extending between said surfaces and away therefrom, opening at one side of the body, and formed by narrowly spaced opposing surfaces, the intersection between the recesses for the electrodes and the slot being constricted to expose to the slot only a small portion of the cross-sectional area of each electrode, and said slot being disposed nonsymmetrically with respect to the axial plane of the electrodes causing increased impingement of the discharge against one of said surfaces.

9. A lightning arrester for protecting electrical apparatus including a pair of electrodes extending generally parallel to one another from their terminal ends and with terminal end portions on at least adjacent sides thereof having outer surfaces sloping approximately 30 to 60 degrees with respect to the axial direction, and a unitary body of insulating material having a discharge slot therein extending between said surfaces and straight forwardly in the axial direction away therefrom to open at one side of the body, and formed by narrowly spaced opposing surfaces, the intersections between the recesses for the electrodes and the slot being constricted to expose to the slot only a small portion of the cross-sectional area of each electrode, and said slot being disposed nonsymmetrically with respect to the axial plane of the electrodes to cause increased impingement of the discharge against one of said surfaces, and having its edges between said surfaces diverging forwardly and reentrantly converging rearwardly to form an approximate arc from electrode to electrode; the insulating body being approximately round in planes transverse to the axial direction, and an elastically prestretched steel band surrounding the insulating body.

10. A lighting arrester for protecting electrical apparatus including a pair of electrodes extending generally parallel to one another from their terminal ends and a unitary body of insulating material having a discharge slot therein extending between said electrodes and straight forwardly in the axial direction away therefrom to open at one side of the body, and formed by narrowly spaced opposing surfaces, the intersections between the recesses for the electrodes and the slot being constricted to expose to the slot only a small portion of the cross-sectional area of each eelctrode, and said slot being disposed nonsymmetrically with respect to the axial plane of the electrodes to cause increased impingement of the discharge against one of said surfaces, and having its edges between said surfaces diverging forwardly and reentrantly converging rearwardly to form an approximate are from electrode to electrode and through the intersections; the insulating body being approximately round in planes transverse to the axial direction, and an elastically prestretched steel band surrounding the insulating body.

11. A lighting arrester for protecting electrical apparatus including a pair of electrodes extending generally parallel to one another from their terminal ends and a unitary body of insulating material having a discharge slot therein extending between said electrode and straight forwardly in the axial direction away therefrom to open at one side of the body, and formed by narrowly spaced opposing surfaces, the intersections between the recesses for the electrodes and the slot being constricted to expose to the slot only a small portion of the cross-sectional area of each electrode, and said slot being disposed nonsymmetrically with respect to the axial plane of the electrodes causing increased impingement of the discharge against one of said surfaces, and having its edges between said surfaces diverging forwardly and reentrantly converging rearwardly to form an approximate are from electrode to electrode and through the intersections.

12. A lightning arrester for protecting electrical apparatus including a pair of electrodes extending generally parallel to one another from their terminal ends and with terminal end portions on at least adjacent sides thereof having outer surfaces sloping approximately 30 to degrees with respect to the axial direction, and a unitary body of insulating material having a discharge slot therein extending between said surfaces and straight forwardly in the axial direction away therefrom to open at one side of the body, and formed by narrowly spaced opposing surfaces, the intersections between the recesses for the electrodes and the slot being constricted to expose to the slot only a small portion of the cross-sectional area of each electrode, and said slot being disposed nonperpendicularly to the electrode surfaces exposed therein to cause increased impingement of the discharge against one of said surfaces, and having its edges between said surfaces diverging forwardly and reentrantly converging rearwardly to form an approximate are from electrode to electrode.

References Cited UNITED STATES PATENTS 3,257,575 6/1966 Milligan 3l3-231 2,334,218 11/1943 Roloson 313231 2,620,452 12/1952 Petersen 313l8 JOHN W. HUCKERT, Primary Examiner B. ESTRIN, Assistant Examiner US. Cl. X.R. 31535, 36 

